1. Field of the Invention
The invention relates to a turbojet venting pipe, to a method for mounting such a pipe and to a turbojet provided with one such pipe.
2. Description of the Related Art
A turbine engine for an aircraft generally comprises, from upstream to downstream in the flow direction of the gases, a fan, one or more compressor stages, for example a low-pressure compressor and a high-pressure compressor, a combustion chamber, one or more turbine stages, for example a high-pressure turbine and a low-pressure turbine, and a gas exhaust nozzle. One turbine may correspond to each compressor, both being connected by a shaft, thus forming for example a high-pressure core and a low-pressure core.
A turbojet generally has, substantially at the upstream end of the high-pressure core, an “upstream compartment” containing components of the rolling bearing and gearing type. It furthermore has, substantially at the downstream end of the high-pressure core, a “downstream compartment” containing components of the rolling bearing and gearing type. These compartments are immersed in an atmosphere containing oil for lubrication of the various components. A gas flow furthermore passes through them, in particular for ventilation purposes. In order to prevent the oil from being transported out of the compartments by the gas flow, the gases are evacuated in “deoilers”, which are generally formed by radial passages formed in the low-pressure shaft and on the wall of which the oil is captured in order to be reinjected into the corresponding compartment, by centrifugal effect. The deoilers communicate with a (likewise rotating) pipe referred to as a venting pipe, in the interior of which the gases are transported from the deoilers in order to be ejected at the exit of the venting pipe, generally at the nozzle of the turbojet.
The venting pipe extends inside the low-pressure shaft, concentrically therewith, the low-pressure shaft for its part extending inside the high-pressure shaft, concentrically therewith. The venting pipe rotates with the low-pressure shaft; it generally extends over a majority of the longitudinal dimension of this shaft. The venting pipe makes it possible to guide the gases and, in particular, to avoid contact of the oil-laden gases with the low-pressure shaft which, owing to the high temperature of the latter, could lead to coking phenomena of the oil in suspension in the gases.
In most known turbojets, the low-pressure shaft has a wall of variable thickness, the internal surface of its wall having a variable diameter along the shaft. The person skilled in the art conventionally refers to a so-called “bottle”-shaped shaft owing to the shape of its internal wall; the internal surface of the wall of such a shaft has a larger diameter in its central region than in its end portions.
The venting pipe generally has a wall of relatively small thickness compared with the thickness of the wall of the low-pressure shaft. Because of its slenderness, it needs a certain number of supports on the internal surface of the wall of the low-pressure shaft, not only at its ends but also in the central part. A mounting problem then arises, since the venting pipe needs to be mounted via an end of the shaft, which has a diameter less than the diameter of its central part but on the internal surface of which the pipe nevertheless needs to bear in order to ensure that it is held. This problem is solved in the prior art by using systems of conical rings and nuts which are mounted in the shaft before the pipe is mounted. These systems are complex and require sufficient clearance between the external surface of the wall of the venting pipe and the internal surface of the wall of the low-pressure shaft.
In certain recent turbojets, the diameter of the high-pressure shaft is reduced relative to that of previous turbojets. The size of the engine therefore requires that a wall of constant thickness be provided for the low-pressure shaft, with external and internal surfaces of constant diameters along the majority of the shaft, these diameters furthermore being reduced relative to those of the shafts of the prior art. The venting pipe must for its part have a diameter substantially equal to that of the venting pipes of the prior art, in order to ensure the discharge of an equivalent gas flow rate. For this reason, the space between the external surface of the venting pipe and the internal surface of the low-pressure shaft is small and makes it difficult to mount points of support. Notwithstanding, the presence of such points of support along the venting pipe remains necessary in view of its slenderness (about 2 meters in length with a diameter of 60 millimeters).